During maternal fasting changes in ovine fetal metabolism occur such that the fetus becomes less reliant on maternally acquired glucose and increasingly reliant on amino acids as an energy source. Placental and hind limb balance studies indicate that the branched chain amino acids may be of central importance to the fetus during fasting. Thus, the proposed investigation will characterize the ovine fetus utilization of the branched chain amino acids and their alpha keto analogues. Utilizing the chronic fetal lamb preparation the following specific questions will be addressed. 1. Does the fetal hind limb utilize completely it's large uptake of branched chain amino acids or are significant quantities of the alpha-ketoacids given off for inter-organ transport? Quantitative balance studies for glucose, lactate, pyruvate, amino acids and branch chain alpha-ketoacids will be determined across the fetal hind limb in fed and fasted animals. 2. How does maternal fasting effect fetal tissue distribution and degree of activity of the enzymes responsible for degradation of the branched chain amino acids? Tissue samples from the in vivo preparations will be obtained from fetal skeletal muscle, liver, kidney, heart, adipose tissue and brain as well as placenta and maternal skeletal muscle and liver. These samples will be assayed for branched chain aminotransferase activity, branched chain keto acid dehydrogenase activity and pyruvate dehydrogenase activity; phosphorylated and non-phosphorylated forms will be quantitated for the latter two enzymes. 3. What is the placental balance of branched chain alpha-ketoacids? Do changes in fetal branched chain amino acid carbon and nitrogen utilization correlate with changes in fetal glucose carbon utilization? Ovine placental balance of the alpha-ketoacids as well as other substrates mentioned above will be quantitated in the fed and fasting state. In addition, using radioactive and stable isotope techniques, fetal glucose carbon and branched chain amino acid carbon utilization will be determined simultaneously in both a fed and fasting state. In summary, the proposed research will characterize alterations in fetal branched chain amino acid metabolism as the fetus adapts to a diminished maternal nutrient supply.